Method and apparatus for cleaning a gas

ABSTRACT

A method and an apparatus for cleaning a gas containing pollutants, such as hydrogen chloride and heavy metals, are described. The polluted gas (2) is contacted with an aqueous washing liquid (3) in a scrubber (1), whereupon part of the washing liquid is preevaporated in a preevaporation unit (7) to form a hydrochloric-acid-containing top fraction, which is essentially free from heavy metals, and a bottom fraction containing hydrochloric acid and heavy metals. Part of the bottom fraction is recirculated (15) for renewed preevaporation, while the remainder of the bottom fraction is treated (16) to remove any heavy metals present. The top fraction from the preevaporation is distilled in a rectification unit (22) to form a top fraction (31) and a bottom fraction (35).

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for cleaninga gas containing pollutants, such as hydrogen chloride and heavy metals,by contacting the gas with an aqueous washing liquid which takes up thepollutants. More specifically, the invention relates to a method and anapparatus for cleaning flue gases formed during the combustion of waste,such as household refuse, and recovery of the pollutants removed duringthe cleaning as valuable products, such as hydrochloric acid, solidchlorides, heavy metals in pure form, and/or minimising the volume ofthe pollutants removed.

DESCRIPTION OF CONVENTIONAL ART

During the combustion of waste, such as industrial waste and householdwaste (refuse), the flue gases will contain, inter alia, hydrogenchloride, heavy metals and other pollutants. The hydrogen chloride isformed e.g. when the burnt waste contains sodium chloride from foodproducts or chloride-containing plastics, such as polyvinyl chloride.Before the flue gases are emitted into the atmosphere, hydrogenchloride, heavy metals and other pollutants must be removed by washingthe flue gases with an aqueous solution in a so-called scrubber orwasher, which results in a diluted aqueous solution of hydrochloricacid. In addition to hydrochloric acid, the aqueous solution containsother pollutants from the flue gases, such as heavy metals, i.e. mainlyzinc, lead and mercury, inert solids, as well as bromides and fluorides.The hydrochloric acid concentration is about 2-15% by weight dependingon, inter alia, the design of the scrubber and the concentration ofhydrogen chloride in the flue gases. Here and in the following, thehydrochloric acid concentration is indicated in per cent by weight ofHCl, based on the total amount of HCl and water.

It is known to remove the heavy metals from the washing liquid by addingchemicals which with heavy metals form sparingly soluble compounds. Oneexample is the adding of sodium sulphide, whereby the heavy metals areprecipitated as heavy metal sulphides. According to today's technique,the heavy metals are not utilised, but the heavy metal sludge producedis a waste product to be disposed of.

Moreover, it is known to remove the absorbed hydrochloric acid from thewashing liquid by distillation. Since hydrochloric acid and water forman azeotrope, a hydrochloric acid is obtained during the distillation,having a maximum concentration of about 20% by weight. Instead ofremoving the hydrochloric acid in the washing liquid by distillation, aprior art measure is the cleaning of the washing liquid in a wastewatertreatment plant and the adding of a chloride-forming substance to thewashing liquid and evaporate this for recovery of the chloride of thewashing liquid as a solid chloride salt. In these contexts,"chloride-forming substance" means substances which form chlorides withchloride ions. More precisely, oxides, hydroxides or carbonates ofalkali metals or alkaline earth metals are intended. Amongchloride-forming substances preferred, NaOH, NaCO₃, CaO, Ca(OH)₂ andCaCO₃ may be mentioned.

As an example of prior art technique in the field, PCT 92/15519 may bementioned, which relates to wet treatment of a gas containing hydrogenchloride, the gas being contacted with a washing liquid which contains achloride-forming substance, such as calcium carbonate. After an optionalpreevaporation step, the washing liquid is conducted to a distillationunit and distilled, after adding sulphuric acid for recovery of highlyconcentrated hydrochloric acid.

EP 0 393 402 is a further example of prior art technique, whichdiscloses cleaning of hydrochloric-acid-containing flue gases byabsorption in water. The absorption is performed in several steps, theconcentration of hydrochloric acid being increased to above 50 g ofHCl/l. The thus obtained hydrochloric-acid-containing aqueous solutionis distilled to separate the hydrochloric acid. As also appears from EP0 393 402, it is known to add carbonates or hydroxides of alkali metalsor alkaline earth metals to the washing liquid, and then evaporate thewashing liquid to recover e.g. sodium chloride or calcium chloride.

Another example of prior art technique is U.S. Pat. No. 4,874,591 whichrelates to treatment of hydrogen-chloride- andsulphur-dioxide-containing waste gases with a solution containingmagnesium ions to remove the hydrogen chloride and the sulphur dioxidein the form of magnesium chloride and magnesium sulphate, respectively.By adding calcium ions, the sulphate is precipitated as calciumsulphate. The magnesium chloride solution is converted by pyrohydrolysisinto hydrochloric acid and magnesium oxide which is then converted intomagnesium hydroxide and recirculated.

A further example of prior art technique is U.S. Pat. No. 4,539,190which discloses flue gas treatment by detecting the amount of hydrogenchloride in the flue gas and by adding a corresponding amount of sodiumsalt solution to convert the hydrogen chloride into sodium chloride, andby adding calcium ions for absorption of sulphur dioxide.

A still further example is U.S. Pat. No. 3,929,968 which relates to amethod for dry collection of waste materials in hot gases, the hot gasfirst being cooled in an evaporative cooler and subsequently supplied toa dry collecting device, such as a bag filter, to remove solidparticles. Then the gas is fed into a scrubber for absorption ofsoluble, acidic or basic components. The solution from the scrubber isthe feeding liquid to the evaporative cooler.

One more example of prior art is DE 39 20 544 which discloses thecleaning of flue gases from hydrogen chloride, sulphur dioxide, heavymetals and other pollutants by means of a wet cleaning system. Thehydrogen chloride is absorbed and bound as sodium chloride which afterevaporation crystallises and is recovered. The sulphur dioxide isabsorbed and precipitated as calcium sulphate, and the heavy metals areprecipitated as e.g. sulphides.

Despite the technique thus known, there is a need for a method forcleaning a gas, preferably flue gas, which contains pollutants, such ashydrogen chloride and heavy metals, the pollutants being efficientlyremoved while the amount of waste products is minimised, said methodpermitting recovery of the hydrogen chloride as high-qualityhydrochloric acid or as chloride salt, and also recovery of the heavymetals in pure form. The cleaning method and the apparatus used thereforshould further be flexible and cost-effective to operate and constructedin such a way that the energy consumption is optimised.

SUMMARY OF THE INVENTION

The present invention aims at achieving one or more of theabove-mentioned objects.

One object of the invention thus is to recover from flue gaseshigh-quality hydrochloric acid.

A further object is to recover from flue gases heavy metals, such aszinc, lead and mercury, in pure form.

Another object is to recover from flue gases polluting hydrogen chlorideas solid chlorides, such as sodium chloride.

A still further object is to recover hydrochloric acid having asuperazeotropic concentration.

Another further object is to provide an economical and flexibleapparatus, by means of which one or more of the objects above can beachieved.

The invention provides a method for cleaning a gas containingpollutants, such as hydrogen chloride and heavy metals, by contactingthe gas with an aqueous washing liquid which takes up the pollutants,characterised by

a) preevaporating a washing liquid with absorbed pollutants, therebyforming a hydrochloric-acid-containing top fraction which is essentiallyfree from heavy metals, and a bottom fraction which containshydrochloric acid, heavy metals and inert material,

b) recirculating part of the bottom fraction, for renewed evaporation,while the remainder of the bottom fraction is treated to remove heavymetals, and

c) distilling the top fraction from the preevaporation step, therebyforming a top fraction and a bottom fraction.

The invention also provides an apparatus for cleaning a gas containingpollutants, such as hydrogen chloride and heavy metals, characterised by

a scrubber having inlets for polluted gas and for a washing liquid, andoutlets for cleaned gas and for a hydrochloric-acid-containing washingliquid,

a preevaporation unit having an inlet for thehydrochloric-acid-containing washing liquid, an outlet for ahydrochloric-acid-containing top fraction and an outlet for aheavy-metal-containing bottom fraction,

a heavy metal separation unit having an inlet connected with the bottomfraction outlet of the preevaporation unit, an outlet for separatedheavy metals, and an outlet for recirculation of an essentiallyheavy-metal-free fluid to the scrubber,

a distillation unit having inlets for the top fraction from thepreevaporation unit and outlets for a top fraction and a bottomfraction.

The further features and advantages of the invention are apparent fromthe following description and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings in which:

FIG. 1 illustrates an embodiment of an apparatus for carrying out themethod according to the invention;

FIG. 2 illustrates a further embodiment of the apparatus according tothe invention;

FIG. 3 illustrates another embodiment of the apparatus according to theinvention; and

FIG. 4 illustrates a still further embodiment of the apparatus accordingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a scrubber 1 having an inlet for polluted gas 2. Asmentioned above, the polluted gas is preferably a flue gas from thecombustion of e.g. industrial waste or household refuse and containspollutants, such as hydrogen chloride, heavy metals (zinc, lead, mercuryetc.), inert particles, nitric oxides, sulphur dioxide etc. In thescrubber, the polluted gas 2 is contacted with a washing liquid 3 whichis an aqueous solution. If required, the washing liquid may contain aneutralisation agent, such as an oxide or hydroxide of an alkali metalor alkaline earth metal, to increase the pH of the washing liquid. Whenthe washing liquid is contacted, it takes up the major part of thepollutants from the polluted gas, and the cleaned gas leaves at 4 forfurther treatment, for example in a SO₂ scrubber, before being emittedinto the atmosphere.

After cleaning the gas, the washing liquid leaves the scrubber via aconduit 5. From the conduit 5 the major part of the washing liquid isbranched off and recirculated to the scrubber via the conduit 6. Theremaining part of the washing liquid which, in addition to heavy metalsand other minor pollutants, contains about 5-15% by weight of HCl andthe balance water, is fed in the conduit 5, optionally via a storagetank (not shown), to a preevaporation unit 7 comprising a heat exchanger8 and a preevaporator 9. The washing liquid reaches the preevaporationunit 7 via the conduit 10, is heated in the heat exchanger 8 by means ofsteam 11 and is conducted from the heat exchanger 8 via the conduit 12to the preevaporator 9. The pressure in the preevaporator 9 suitably isabout 0.7-0.9 bar. In the preevaporator 9, the washing liquid is dividedinto a top fraction in the form of steam, which contains about 5-15% byweight of HCl and the balance water, and a liquid bottom fraction. Thebottom fraction leaves the preevaporator 9 via the conduit 13 and isdivided into a first partial flow which is recirculated via the conduit14 to the heat exchanger 8, and a second partial flow which is fed viathe conduit 15 to a heavy metal separation unit 16. The second partialflow which is branched off via the conduit 15 is typically about 1-5% ofthe flow which is supplied in the conduit 5 from the scrubber 1 to theheat exchanger 8 and contains about 15-20% by weight of HCl, about 5-10%by weight of heavy metal cations and inert solids, and the balance(about 0-75% by weight) water.

The heavy metal separation unit may be designed in various ways. In caseone merely wants to separate the heavy metal pollutants as a wasteproduct to be disposed of, the heavy metal separation unit may consistof a film evaporation unit from which the heavy metal pollutants areseparated as a sludge via the outlet 17. Alternatively, the heavy metalseparation unit may be a device for precipitating the heavy metalpollutants by adding e.g. a sulphide compound, such as sodium sulphide,whereupon heavy metal sulphides formed are separated by sedimentation,centrifugation or in some other manner and removed as a sludge via theoutlet 17. The fluid (steam or liquid) which is obtained in theseparation of the heavy metal pollutants from the bottom fraction andwhich contains about 20% by weight of HCl and the balance water isbranched off from the heavy metal separation unit via a conduit 18 andrecirculated to the scrubber 1.

In case one wants to recover and utilise the heavy metal pollutants inthe bottom fraction from the preevaporation unit 7, which constitutes apreferred embodiment of the invention, the heavy metal separation unit16 comprises a device for recovering the heavy metals in pure metallicform via the outlet 17. Such a device preferably consists of anelectrolyser for electrolytic precipitation of the heavy metals, or anion exchanger for separating the heavy metals by ion exchange. Thespecific conditions for such recovery of heavy metals by electrolysis orion exchange are per se known or can easily be obtained by the expert bymeans of simple routine experiments. In this manner, the heavy metals(zinc, lead, mercury) may be recovered in pure form and utilised. Itwill be appreciated that this brings an advantage compared with thedisposal thereof as waste, with the risk of the environment beingpolluted. Also in this embodiment, the liquid separated from the heavymetals may be subjected to further treatment, for example in a filmevaporator, as described above, to separate the remaining pollutantswhich are removed via another outlet (not shown), whereuponrecirculation to the scrubber 1 is effected via the conduit 18.

The top fraction in the form of steam leaves the preevaporator 9 via aconduit 19 and is condensed in a cooler 20 and subsequently supplied asfeed via a conduit 21 to a rectification unit 22 which comprises arectification column 23, a heat exchanger 25 heated with steam 24, and acondenser 27 cooled with water 26. The top fraction from thepreevaporation unit, which is fed as inflow to the rectification unitvia the conduit 21, mainly contains about 5-15% by weight of HCl and thebalance water. The top fraction obtained in the distillation leaves therectification column via a conduit 28, is condensed in the condenser 27,leaves the condenser via a conduit 29 and is divided into a partial flowwhich is recirculated to the rectification column via a conduit 30, anda further partial flow which leaves as a product flow via a conduit 31.The product flow in the conduit 31 consists of an aqueous solution freefrom heavy metals and having a concentration of HCl of up to about 15%by weight, depending on the conditions selected for the distillation inthe rectification column 23. If one thus chooses to take out from thecolumn a small amount of a bottom product, which has a high (azeotropic)concentration of hydrochloric acid, a top fraction rich in hydrochloricacid is obtained, and vice versa. When taking out a large amount of theazeotropic bottom product, a top fraction almost free from hydrochloricacid is obtained, which may be used as make-up water for the scrubber 1.When taking out a smaller amount of the azeotropic bottom product, anessentially pure, diluted hydrochloric acid which is free from heavymetals is obtained as the top fraction, having for example aconcentration of about 5-15% by weight. This pure hydrochloric acid maybe used in different contexts which do not require highly concentratedhydrochloric acid, for example when regenerating an ion exchange systemfor treating feed water for steam boilers.

A bottom product leaves the rectification column 23 via a conduit 32, apartial flow being recirculated to the rectification column via aconduit 33, the heat exchanger 25 and a conduit 34. The remaining partof the bottom fraction is taken out as a product at 35 and consists of ahydrochloric acid solution, whose concentration of hydrochloric aciddepends on the selected distillation conditions stated above. Theproduct flow 35 is typically about 25-45% of the amount of the washingliquid, which is fed from the scrubber to the preevaporation unit 7.

By using the embodiment of the invention as described above, it ispossible to clean flue gases from polluting hydrogen chloride and heavymetals and recover the hydrogen chloride as hydrochloric acid having aconcentration of up to azeotropic concentration (about 20% by weight).The heavy metals (zinc, lead and mercury) may be recovered in pure form.On the whole, the waste which is to be disposed of will be minimised.

FIG. 2 illustrates a different embodiment of the apparatus according tothe invention. This embodiment conforms in many respects with the one inFIG. 1 which has been described above. Corresponding items havetherefore been given equal numerals, and for the description of theseitems, reference is made to the description in connection with FIG. 1.

The main difference between the embodiment in FIG. 1 and the one in FIG.2 is that in the latter, at least part of the product flow 35 isneutralised with a chloride-forming substance, such as NaOH or Na₂ CO₃,and is recovered as a solid chloride product. As shown in FIG. 2, partof the product flow of hydrochloric acid is branched off via a conduit36, and the chloride-forming substance is added to this branched-offhydrochloric acid via a supply station 37. The flow is then fed via aconduit 38 to an evaporation and crystallisation unit 39 which comprisesa heat exchanger 40, an evaporator 41 and a condenser 42. The flowsupplied in the conduit 38 to the evaporation unit mainly consists ofabout 25-30% by weight of chloride, such as sodium chloride, and thebalance water. After heating in the heat exchanger 40, thechloride-containing supply flow is fed via a conduit 43 to theevaporator 41. A concentrated, chloride-containing bottom product leavesthe evaporator via a conduit 44, from which a partial flow isrecirculated to the evaporation unit via a conduit 45 and the conduit38, while the remainder of the bottom fraction is fed via a conduit 46to a unit 47, such as a centrifuge, for separating solid chloride,preferably sodium chloride, which leaves as a product flow 48, while theeffluent which is freed from solid chloride is recirculated via aconduit 49 to the evaporation unit 39.

Via a conduit 50, a top fraction in the form of water vapour leaves theevaporator 41, said water vapour being condensed in the condenser 42 towater which leaves the condenser as a product flow 51. The condensatewater from the product flow 51 may be used e.g. as make-up water for thewashing liquid in the scrubber 1, as indicated by arrow 52.

By means of the embodiment in FIG. 2 as described above, it is possiblein the cleaning of hydrogen-chloride-containing flue gas to recover thehydrogen chloride both as hydrochloric acid and as solid chloride,preferably sodium chloride. Like in the embodiment in FIG. 1, heavymetal pollutants can either be separated for disposal thereof or berecovered in pure form.

FIG. 3 illustrates a further embodiment of an apparatus for carrying outthe invention. The apparatus conforms in many respects with thosedescribed above in FIGS. 1 and 2, and corresponding items have beengiven equal numerals. These items are not described here in detail, butreference is made to the description given in connection with FIGS. 1and 2.

The apparatus in FIG. 3 distinguishes from the one in FIG. 2 byrecovering as a top product in the distillation in the rectificationcolumn 23 a high-concentration hydrochloric acid, i.e. having aconcentration above the azeotropic one. This is achieved by supplying,in addition to the flow of low-concentration hydrochloric acid, in theconduit 21 to the rectification column 23 a flow of calcium chloride. Asillustrated in FIG. 3, the bottom product leaves the rectificationcolumn 23 via the conduit 32, a partial flow is branched off andrecirculated to the column via the conduit 33, while the remainder ofthe bottom fraction is fed via the conduit 36 to the evaporation unit39. In this embodiment, preferably no product flow 35 of the bottomfraction is branched off. Adjacent a supply station 37, the bottomfraction flow in the conduit 36 is supplied with an addition of calciumchloride, and the mixture is supplied to the evaporation unit 39 via theconduit 38. The calcium-chloride-containing flow in the conduit 36mainly contains about 20-40% by weight of calcium chloride and thebalance water. Instead of being arranged on the conduit 36, the supplystation 37 can be arranged on the conduit 38. The bottom fraction flowand the added calcium chloride is heated in the heat exchanger 40 fromwhich it is conducted to the evaporator 41 in which it is concentratedand leaves as a concentrated flow of calcium chloride via the conduit44, 45. This flow is recirculated to the evaporation unit 39, a partialflow having a concentration of calcium chloride of about 50-60% byweight being branched off via a conduit 53 and supplied as feed to therectification column 23. By the simultaneous distillation of thisconcentrated calcium chloride solution and the hydrochloric acidsolution supplied via the conduit 21 a superazeotropic hydrochloric acidis produced as a top fraction from the rectification column 23 andbranched off via the conduit 28 to be condensed in the condenser 27. Thecondensed, highly concentrated hydrochloric acid which is branched offfrom the condenser via the conduit 29 has a concentration ofhydrochloric acid of about 30-95% by weight. Some of this hydrochloricacid solution is recirculated via the conduit 30 to the column 23, whilethe remainder is fed via the conduit 31 to an absorption and coolingunit 54. In this unit, water is added, at 55, to the highly concentratedacid which leaves as a hydrochloric acid solution product, at 56, havinga concentration of hydrochloric acid of about 30% by weight or more.

The evaporation of the calcium-chloride-containing bottom fraction inthe evaporator 41 results in a top fraction in the form of steam, whichmainly consists of water and which is branched off via the conduit 50 tobe condensed in the condenser 42 and branched off therefrom ascondensation water 51. As mentioned above in connection with FIG. 2,this condensation water may be used as makeup water in the scrubber 1(see arrow 52).

To separate superazeotropic hydrochloric acid as the top fraction in thecolumn 23, it is preferred to use calcium chloride in the feed flow 53to the column, but it is also possible to use sulphuric acid instead ofcalcium chloride.

By means of the embodiment according to FIG. 3 as described above, it ispossible according to the invention to recover from ahydrogen-chloride-containing flow of gas the hydrogen chloride as ahighly concentrated (superazeotropic) hydrochloric acid. Any heavymetals present may, like in FIGS. 1 and 2, either be separated, forexample as heavy metal sulphides, and disposed of, or be recovered inpure form.

FIG. 4 illustrates a further embodiment of the apparatus according tothe invention. The apparatus conforms in many respects with thosedescribed above and illustrated in FIGS. 1-3, and corresponding itemshave been given equal numerals and will now not be described, butreference is made to the description given above in connection with therespective Figures of these items.

The embodiment according to FIG. 4 distinguishes from that in FIG. 3 bythe fact that the highly concentrated hydrochloric acid which is the topfraction from the rectification column is not recovered as an acid butas a solid chloride salt.

Like in the embodiment according to FIG. 3, the top fraction of highlyconcentrated hydrochloric acid is supplied via the conduit 31 to theabsorption and cooling unit 54 in which water is added at 55 to obtain ahydrochloric acid having a concentration of about 50-80% by weight,which leaves via the conduit 56. To this hydrochloric acid achloride-forming substance, such as NaOH or Na₂ CO₃, is added via a feed57 for precipitation of a solid chloride product (sodium chloride). Inpractice, the feed 57 preferably is a mixing tank. The precipitatedchloride product is then separated in a separating unit 58, whichpreferably is a centrifuge, and subsequently leaves this as a productflow 59 of solid chloride (sodium chloride).

It will be appreciated that modifications of the apparatus and methodsaccording to FIGS. 1-4, as described above, are obvious to and may bemade by the man skilled in the art, and such modifications are alsocomprised by the invention. Thus, it will be appreciated e.g. that theapparatus according to FIG. 4, slightly modified, can be operated asdescribed above in connection with FIGS. 1-3, and the apparatus andmethod according to FIG. 4 therefore is the most flexible, complete and,consequently, preferred embodiment of the invention.

To further elucidate the invention, some non-restrictive examples followbelow.

EXAMPLE 1

Flue gas from a municipal solid waste incineration plant was treated ina wet scrubber system as described above in connection with FIG. 1. Fromthe wet scrubber, a washing liquid flow of about 450 kg/h, containingapproximately 7% by weight of HCl and the balance mainly H₂ O, wasdiverted to the preevaporation unit. From the preevaporator a bottomproduct of 22 kg/h was taken out to a settling tank where the settledresidue was taken out and treated in a heavy metal separation unit forseparating the heavy metals as Hg, Zn and Pb, using conventionaltechnology. The liquid from the settling tank was recycled to thepreevaporation unit. The remaining solution after treatment in the heavymetal separation unit, which solution mainly comprised H₂ O, HCl andinert material, was further treated in a film evaporation unit beforebeing recycled to the scrubber.

The top vapour product from the preevaporation unit amounted to 428 kg/hand contained 6.6% by weight of HCl and 93.3 by weight of H₂ O. Aftercondensing, this acid was further treated in a rectification unit.However, the acid may be used directly as a product where a very pureacid is not required, e.g. as a pickling acid in the iron and steelindustry.

The condensed acid from the preevaporation unit was treated in therectification unit at a pressure of 0.8 bar and the correspondingtemperature. About 142 kg/h of a bottom product containing about 19% byweight of HCl and about 286 kg/h of a top product were produced. The topproduct was condensed and a liquid of about 99.5% by weight of H₂ O wasobtained. This product may be used as make-up water in the scrubber.

EXAMPLE 2

An apparatus as described in connection with FIG. 4 was used. Thescrubber, the preevaporation unit and the heavy metal separation unitwere operated as described in Example 1. However, the CaCl₂ solutionobtained from the evaporation unit downstream of the rectification unitwas circulated to the rectification column and distilled together withthe top condensate feed from the preevaporation unit. The distillationwas carried out in the rectification unit at a pressure of about 0.7 barand the corresponding temperature. About 60 kg/h of 45% by weight of HClwas obtained as the top product from the rectification unit. The acidwas neutralised, using pure NaOH, and NaCl was crystallised andseparated in a centrifuge. The quality of the NaCl product obtained wasfound to be equal to or even better than that produced by conventionaltechnology.

I claim:
 1. Method for cleaning a gas containing pollutants, includinghydrogen chloride and heavy metals, by contacting the gas with anaqueous washing liquid which takes up the pollutants, comprising thesteps of:a) preevaporating the washing liquid and the pollutantstaken-up, thereby forming a hydrochloric-acid-containing top fraction,which is essentially free from heavy metals, and a bottom fractioncontaining hydrochloric acid, heavy metals, and inert material, b)recirculating part of the bottom fraction from step a), for renewedpreevaporation, while the remainder of the bottom fraction from step a)is treated for removal of heavy metals and recycled to a scrubber, andc) distilling the top fraction from the preevaporation step to form atop fraction and a bottom fraction.
 2. Method as claimed in claim 1,wherein the remainder of the bottom fraction in step b) is treated forrecovery of heavy metals in pure form.
 3. Method as claimed in claim 1,wherein the bottom fraction from step c) contains hydrochloric acid, atleast part of the bottom fraction from step c) is supplied with achloride-forming substance and concentrated by evaporation, anddeposited chloride is separated therefrom.
 4. Method as claimed in claim1, wherein a chloride salt is added to the bottom fraction from step c),the chloride-containing bottom fraction is concentrated by evaporation,and at least part of the concentrated, chloride-containing bottomfraction is distilled in step c) together with the top fraction from thepreevaporation step to form a top fraction having a concentration ofhydrochloric salt which exceeds the azeotropic concentration.
 5. Methodas claimed in claim 4, wherein the top fraction from step c) containshydrochloric acid and is supplied with a chloride-forming substance forprecipitation and separation of solid chloride.
 6. Apparatus forcleaning a gas containing pollutants, including hydrogen chloride andheavy metals, which comprises:a scrubber having inlets for polluted gasand for a washing liquid, and outlets for cleaned gas and forhydrochloric-acid-containing washing liquid, a preevaporation unithaving an inlet for the hydrochloric-acid-containing top fraction, anoutlet for a hydrochloric-acid containing top fraction, and an outletfor a bottom fraction containing heavy metals, a recirculation conduitconnected with the bottom fraction outlet of the preevaporation unit andwith the inlet of the preevaporation unit, a heavy metal separation unithaving an inlet connected with the bottom fraction outlet of thepreevaporation unit, an outlet for separated heavy metals, and an outletfor recirculation of a fluid, which is essentially free from heavymetals, to the scrubber, a rectification unit having inlets for the topfraction from the preevaporation unit and outlets for a top fraction anda bottom fraction.
 7. Apparatus as claimed in claim 6, wherein theoutlet for the bottom fraction from the rectification unit is connectedwith a station for supplying a compound of a chloride-forming substance,and with the inlet of an evaporation unit which has outlets for waterand a concentrated bottom fraction, respectively.
 8. Apparatus asclaimed in claim 7, wherein the outlet for the concentrated bottomfraction from the evaporation unit is connected with a unit forseparating solid chloride.
 9. Apparatus as claimed in claim 7, whereinthe rectification unit has an inlet for the concentrated bottom fractionfrom the evaporation unit.
 10. Apparatus as claimed in claim 9, whereinthe top fraction outlet of the rectification unit is connected to a unitfor supplying a chloride-forming substance and separating solidchloride.
 11. Method as claimed in claim 2, wherein the bottom fractionfrom step c) contains hydrochloric acid, at least part of the bottomfraction from step c) is supplied with a chloride-forming substance andconcentrated by evaporation, and deposited chloride is separatedtherefrom.
 12. The apparatus of cliam 7, wherein the chloride-formingsubstance is an alkali metal or alkaline earth metal.